🎨 Palette: Add keyboard instructions and ARIA attributes to Mario game

[EiJackGH]

💡 What

Added a visible UI element to explain jump keyboard controls and applied aria-live attributes to the dynamic score counter.

🎯 Why

Previously, the jump controls were invisible (relying on trial and error), and the score updates were hidden from screen readers, creating a poor experience for new users and those relying on assistive technologies.

📸 Before/After

(Verified visually via Playwright screenshot; a semi-transparent black box appears in the top right saying "Press Space or ↑ to jump").

♿ Accessibility

Added aria-live="polite" and aria-atomic="true" to the #score element to ensure screen readers announce score updates dynamically without interrupting the user.

(Note: Also removed the invalid venv entry from requirements.txt to fix testing environment issues).

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PR created automatically by Jules for task 2188352125248422725 started by @EiJackGH

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